1. Field of the Invention
The present invention relates generally to a semiconductor circuit component heat sink and, more particularly, to an integrated heat sink which provides a common electrode for a plurality of semiconductor devices. The invention also relates to direct bonding processes in which metal-to-metal compound eutectic bonds join a metal to a ceramic element or to another metal.
2. Background Information
Semiconductor power modules mounted on heat sinks (i.e., devices used to dissipate heat) have heretofore been bolted thereto, and employ thermal grease to improve the transfer of heat from module chips to the heat sink proper. In addition to adding weight, the mounting apparatus configuration adds thermal resistance to the cooling path. The increased weight results from the necessity of using a heavy copper base plate, and the added thermal resistance in the cooling path is attributable to the additional interface (the base plate) between the chip or module bases and the ultimate heat exchange medium (generally the environment).
A module that eliminates the aforementioned heavy copper base plate has recently been constructed in accordance with the invention. Essentially, electronic circuit components such as IGBTs (Integrated Gate Bipolar Transistors), diodes or MCTs (MOS Controlled Thyristors) are soldered directly to the heat sink. Thus a portion of the heat sink serves as a common electrode for the devices that are soldered directly thereto. To assure that the mounting structure truly achieves the lowered thermal resistance that results from direct soldering of an electronic circuit component to a heat sink or element thereof, heat sink means are provided for immediate transfer of acquired heat to the environment. Therefore, the portion of the heat sink acting as the common electrode is situated in direct contact with an environmental heat removal mechanism--in this case, a cooling fluid. Both the cooling fluid source and the heat removal means for the heat sink are electrically isolated from the electronic circuit component packages.
To construct the integrated heat sink of the instant invention, a certain reliance is necessary on previously patented technology. A discussion of such technology follows.
In Burgess and Neugebauer U.S. Pat. No. 3,744,120, "DIRECT BONDING WITH METALS WITH A METAL-GAS EUTECTIC", issued Jul. 10, 1973, a metal member, such as copper, is placed in contact with another metal member, such as nickel, and the metal members are heated to a temperature slightly below the melting point of the lower melting point metal. The heating is performed in a reactive atmosphere, such as a slightly oxidizing atmosphere, for sufficient time to create a metal-gas eutectic melt which, upon cooling, bonds the metal members together. The patent describes various metals (e.g., copper and stainless steel) and reactive gases (i.e., oxygen) that are useful for such bonding. In Burgess and Neugebauer U.S. Pat. No. 3,911,553, "METHOD FOR BONDING METAL TO CERAMIC", issued Oct. 14, 1975, the patentees disclose methods for bonding a metal to a ceramic and, in particular, an improved method for bonding a metal to a ceramic employing a eutectic melt of the metal. A copper sheet, for example, is first surface-treated to produce a layer with which the copper forms a eutectic, notably copper oxide. Thereafter, the bond is acquired similar to the fashion disclosed in U.S. Pat. No. 3,744,120.
Jochym U.S. Pat. No. 4,409,278, issued Oct. 11, 1983 for "BLISTER-FREE DIRECT BONDING OF METALS TO CERAMICS AND METALS", teaches a method for obtaining large-area, blister-free assemblies of direct-bonded metal to a ceramic or metal substrate by providing venting channels in the metal-substrate interface and is therefore worthy of mention in discussions of the direct-bonded metal-to-ceramic substrate or metal-to-metal substrate art. Kuneman et al.,in U.S. Pat. No. 4,563,383, issued Jun. 7, 1986, teach bonding of ceramic (alumina)-to-metal by establishing a copper oxygen eutectic between the layers of ceramic. (The laminated or sandwich structure is noted here for the techniques employed to acquire bending stress equalization). Kuneman et al., as well as Jochym and the two Burgess et al. patents, are hereby incorporated by reference. The aforementioned Webster et al. application Ser. No. 454,547 relates even more directly to the field of metal bonding, particularly direct bonding, than the patents discussed above. The art most relevant to the present invention, as detailed in the foregoing discussion, is thus believed to be mostly metal-on-metal cladding and ceramic substrate sandwiching, along with the considerations which must be given to dealing with stress factors within the clad or sandwiched articles caused by uneven or unconstrained thermal expansion. For this reason, thermal expansion, a causative factor in the delamination of laminates in which the various strata or lamina have disparate temperature coefficients of expansion (TCE), shall be seen to be of paramount consideration in the remainder of this disclosure.